To initiate and carry out a transmission gear shift, it is first necessary to determine the speed of the drive engine at which the shift should begin, known as the shift speed, and the target gear into which the shift should be made from the gear currently actually engaged. For this purpose, it is usual for operating parameters of the motor vehicle, such as the current driving speed, the current acceleration or deceleration of the motor vehicle, the current rotation speed of the drive engine (engine speed), the load condition or torque of the drive engine, the accelerator pedal position, the change or rate of change of the accelerator pedal position, the actuation of a kick-down switch; if necessary, also the actuation of a vehicle brake, the current driving resistance, in particular in the case of commercial vehicles also the loading condition or mass of the motor vehicle and the inclination of the road, either to be determined directly by corresponding sensors or calculated by the evaluation of control magnitudes, in particular of the drive engine, and/or of the measured values of other operating parameters.
In determining the shift speed and the target gear, a distinction is generally made between a “shift-speed-orientated shift” and a “target-speed-orientated shift”.
For a “shift-speed-orientated shift” the shift speed, i.e., the engine speed at which the gear shift should take place or be initiated, is first determined as a function of relevant operating parameters. Then a gear step appropriate for the momentary driving situation and, in particular the momentary vehicle acceleration is determined and hence, starting from the gear currently engaged, the target gear to which the transmission is to be shifted is identified.
The advantage of a “shift-speed-orientated shift” consists in the relatively spontaneous reaction of the automated stepped gear transmission to changing operation conditions and to commands input by the driver. For example, when moving from flat ground onto a rising slope with a corresponding increase of the driving resistance and sometimes a deceleration of the motor vehicle, a downshift is initiated relatively quickly. Likewise, when the kick-down switch is actuated, via the accelerator pedal, to initiate the kick-down function, an immediate downshift takes place as expected by the driver, since in such a case the current engine speed is regarded as the shift speed. In motor vehicles for whose stepped gear transmissions more than one shift program is available, for example an ecological mode and a sporty mode, the shift speed and the target gear are also influenced by the shift program that is active at the time. Thus, when the ecological mode is active, to achieve low fuel consumption the shift speed is generally lower than in the sporty mode, in the latter of which the achievement of high drive dynamics is paramount.
In contrast, in a “target-speed-orientated shift” the target speed, i.e., the engine speed that should exist after a shift operation, is determined first. Starting from this, a suitable gear step from the current actually engaged gear, and hence the target gear and the shift speed, are determined as a function of current operating parameters, in particular the acceleration of the vehicle. The advantage of a “target-speed-orientated shift” consists in the possibility of being able to operate the drive engine within an optimum speed range, in particular one that is favorable for consumption, under any operating conditions.
A method for a “target-speed-orientated shift” is described in DE 196 38 064 C2. In that document, for an upshift to the next-higher gear it is provided that to reach the same target gear, when driving with low load and high acceleration a shift is initiated at a lower shift speed and, when driving with a high load and low acceleration, it is initiated at a higher shift speed.
In both shift types, respective speed limits must be complied with, namely in a “shift-speed-orientated shift”, a target speed limit and in a “target-speed-orientated shift”, a shift speed limit, in order to keep the engine speed and therefore the desired operating behavior of the drive engine within a desired range, for example with low fuel consumption and/or a high traction force of the drive engine. In extreme cases, these speed limits can be identical to the engine speed limits, i.e., the acceptable minimum speed and the acceptable maximum speed of the drive engine, however, they are usually inside those engine speed limits.
Since in the known methods for controlling gear shifts of automated stepped gear transmissions, in each case only one shift type is envisaged. The possibilities for adapting to the respective existing driving or operating situation are insufficiently utilized. Thus, the purpose of the present invention is to indicate a method of the type mentioned at the start, with which gear shifts, in particular the determination of shift speed and target gear, can be better adapted to the driving or operating situation at the time.